The image-forming processing based the electrophotographic system forms images by using an inorganic photosensitive material such as selenium, selenium alloy, cadmium sulfide or amorphous silicon or by using an organic photosensitive material formed by using a charge-generating agent and a charge-transporting agent.
Namely, the photosensitive material is positively or negatively charged depending upon the charging characteristic of the photosensitive material and is, thereafter, exposed to image-bearing light. The electrostatic latent image formed by the exposure to image-bearing light is developed by using a toner to form a toner image which is then transferred onto a paper or a plastic film and is fixed to obtain a visible image.
When a charge image (portion irradiated with light) formed by the exposure to image-bearing light is to be developed into a visible image, use is made of a toner which is electrically charged to a polarity opposite to the charged polarity of the photosensitive material. This developing system is called normal development. When a portion (portion irradiated with light) from which the electric charge is removed by the exposure to image-bearing light is to be developed into a visible image, use is made of a toner which is electrically charged to the same polarity as the charged polarity of the photosensitive material. This developing system is called reversal development.
The toner is constituted by dispersing a coloring agent and other additives in a binder resin. As an additive for the toner, there has been known a charge control agent that is used for improving charging properties (rate of charge, charge level, charge stability, etc.).
As the charge control agent used for imparting frictional positively charging property to the toner, there have been known Nigrosine dye, azine dye, copper phthalocyanine pigment, quaternary ammonium salt and a polymer having a quaternary ammonium salt on the side change thereof. Further, as the charge control agent used for imparting frictional negatively charging property to the toner, there have been known a metal complex of monoazo dye, metal complexes of salicylic acid, naphthoic acid and dicarboxylic acid, copper phthalocyanine pigment, and resin containing acid components.
For the colored toners of which the market is expected to expand in the future, it is essential to use a light-colored or, desirably, colorless charge control agent that does not affect the hue. As the light-colored or colorless charge control agent, there can be exemplified metal complex compounds of hydroxybenzoic acid derivatives (e.g., see patent documents 1 to 3), aromatic metal dicarboxylate compounds (e.g., see patent document 4), metal complex compounds of anthranilic acid derivatives (e.g., see patent documents 5 and 6), organoboron compounds (e.g., see patent documents 7 and 8), biphenol compounds (e.g., see patent document 9), calix(n) arene compounds (e.g., see patent documents 10 to 15), and cyclic phenol sulfides (e.g., see patent documents 16 to 18), which are for use with the negatively charging toners. For use with the positively charging toners, there can be exemplified quaternary ammonium salt compounds (e.g., see patent documents 19 to 21).